Yes; the air does vary in weight at different times.
How can we tell when the air changes in weight?
By means of a Barometer.
What is a barometer?
It is an instrument having a tube filled with quicksilver.
When the air is heavier, how does it affect the quicksilver?
The quicksilver rises in the tube.
When the air is lighter, how does it affect the quicksilver?
The quicksilver sinks in the tube.
What is the use of the barometer?